Evolution of the semiconductor manufacturing industry is placing greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to decrease time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Electron beams are used in a number of different applications during semiconductor manufacturing. For example, electron beams can be modulated and directed onto an electron-sensitive resist on a semiconductor wafer, mask, or other workpiece to generate an electron pattern on the workpiece. Electron beams also can be used to inspect a wafer by, for example, detecting electrons emerging or reflected from the wafer to detect defects, anomalies or undesirable objects.
These inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and, thus, higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary because even relatively small defects may cause unwanted aberrations in the semiconductor devices.
As semiconductor devices become smaller, it becomes more important to develop enhanced inspection and review tools and procedures to increase the resolution, speed, and throughput of wafer and photomask/reticle inspection processes. One inspection technology includes electron beam-based inspection such as use of a scanning electron microscope (SEM). An SEM uses an electron source. A typical SEM has an electron beam column that includes an electron source to generate one or more electron beams and electron beam elements to focus or deflect the electron beams across a wafer, which is held on a movable support.
Lanthanum hexaboride (LaB6) emitters, Schottky thermal field emitters (TFE), or tungsten cold field emission (CFE) emitters are commonly used as electron sources. These electron sources provide reliable sources of electrons and show good long term stability, noise figure, and brightness. However, they are unable to meet resolution and throughput requirements in the semiconductor industry.
Electron sources can be divided into three broad groups: thermionic sources, field emission sources, and photocathodes. Thermionic sources are usually made of tungsten or lanthanum hexaboride. In thermionic emission, electrons are boiled off the material surface when the electron thermal energy is high enough to overcome the surface potential barrier. Even though thermionic emitters are widely used, they typically require elevated temperatures (e.g., >1300 K) to operate, and may have several drawbacks such as inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness. The demand for more efficient electron sources has driven the research and development of Schottky emitters and cold field electron sources.
In the Schottky emitters, thermionic emission is enhanced by effective potential barrier lowering due to the image charge effect under an applied external electric field. Schottky emitters are typically made of a tungsten wire having a tip coated with a layer of zirconium oxide (ZrOx), which exhibits a low work function (e.g., approximately 2.9 eV). Schottky emitters are currently used in some electron beam systems. Despite being quite successful, thermally-assisted Schottky emitters still need to be operated at high temperature (e.g., >1000 K) and high vacuum (e.g., approximately 10−9 mbar), and have wider than desirable electron emission energy spread due to the high operating temperature.
Cold electron sources, particularly electron field emitters, have been used in field emission displays, gas ionizers, x-ray sources, electron beam lithography, and electron microscopes, among other applications. Field emission takes place when the applied electric field is high enough to reduce the potential barrier on the tip-vacuum interface so that electrons can tunnel through this barrier at a temperature close to room temperature (e.g., quantum-mechanical tunneling). A typical field-emitter comprises a conical emitter tip with a circular gate aperture. A potential difference is established across the emitter cathode, the gate and the anode under an applied external field, resulting in high electric field at the surface of the tip. Electrons tunnel through the narrow surface barrier and travel towards an anode, which is biased at a higher potential than the gate. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitters.
Field emitters, because they can operate near room temperature, have lower energy spread than Schottky and thermionic emitters, and can have higher brightness and electron current than thermionic emitters. However, in practical use, the output current of a field emitter is less stable because contaminants can stick to the tip of the emitter and raise its work function, and hence lower the brightness and current. Additionally, adsorption or desorption of these contaminants from the surface of the emitter tips over time can cause the work function to fluctuate and can lead to instability in the beam current. Periodic flashing (i.e., temporarily raising the tip temperature) is required to remove those contaminants. While the tip is being flashed, the instrument is not available for operation. Instruments in the semiconductor industry are required to operate continuously and stably without interruption, so Schottky emitters are usually used in preference to cold field emitters.
Previous field emitter arrays (FEAs) had multiple conically shaped electron emitters arranged in a two-dimensional periodic array. These field emitter arrays can be broadly categorized by the material used for fabrication into two categories: metallic field emitters and semiconductor field emitters.
Photocathodes also have been used to generate electron beams. A single light beam incident on a photocathode system can generate a single electron beam with high brightness that is capable of delivering high electron current density. However, a problem with single electron beam systems is that even with high brightness systems, single electron beam systems still have relative low throughput for inspection. Low throughput is a drawback to electron beam inspection. With current available electron beam sources, thousands of beams would be required.
For many electron emitters, silicon is a good candidate material for making nanotips because of well-established silicon microfabrication techniques. However, silicon emitters are highly susceptibility to oxidation, which converts the emitter tip to a silicon oxide. The silicon oxide will render the tip inoperable for electron emission due to the high work function of the silicon oxide. Stability also is affected by presence of silicon oxide on the emitter.
Emitters with small tip diameters (e.g., 100 nm or less) used for electron emission also are affected by vacuum conditions. The vacuum conditions can deteriorate field emission performance. Typical electron emitters do not have protective coating to protect from oxidation or carbon build up. A carbon layer grows on the surface of the cathode tips during electron beam emission under ultra-high vacuum (UHV) conditions. Oxidation of surfaces in UHV environments is also likely. Previous emitter designs also were not robust to cleaning of, for example, oxidation or carbon layers.
Schottky thermal field emitters are sensitive to environmental vacuum conditions and require vacuum levels of 10−9 Torr or better for stable operation. Schottky thermal field emitters also have limited brightness and energy spreads of approximately 0.7 eV. Demands on the emitter brightness, virtual source size, and energy spread are not met with these conventional sources for inspection systems as the defect sizes get smaller. In addition, Schottky emitters operate at 1800 K and tungsten CFE emitters have to be periodically cleaned by flashing at temperatures of up to 1000° C. to clean surface adsorbate contamination, and this heat becomes a problem in an array format because it causes thermal drift of the precise placement of the emitter to extractor alignment that is needed.
For faster inspection of wafers and reticles, defect inspection systems comprising arrays of electron beam columns become an attractive alternative to inspection systems comprising a single column because having multiple columns that operate in parallel reduces the overall inspection time required. For such systems with multiple column, such as those with up to hundreds or thousands of columns that operate at the same time, there is a need for electron sources that can be made in arrays using batch-manufacturing techniques. In these arrays, each emitter should have nearly identical properties, geometries, and performance.
Cold field emission electron sources from emitters with nanoscale diameter tips can produce beams of electrons with high brightness and low energy spread (e.g., 0.3 eV). As a result of their high brightness and low energy spread, electron beams from cold field electron sources can be focused into small spots with high current densities. Many different materials can be used to make cold field emitters, including silicon. A major limitation of cold field emission is the difficulty of making the electron beam current stable. The stability of the current produced by cold field emitters is orders of magnitude lower than the stability of more widely-used Schottky electron sources (a thermal field emitter made of tungsten coated with zirconium oxide). Stability in cold field emitters cannot be easily solved simply with improved vacuum. Cold field emitters made of a wide range of materials produce unsteady current even at vacuum as low as 10−11 Torr. The brightness of cold field emitters can be up to two orders of magnitude greater than the brightness of Schottky emitters, and cold field emitters typically have lower energy spread than Schottky emitters, so cold field emitters would be a valuable alternative as an electron source to Schottky emitters for a wide range of applications if their emission stability could be improved.
Cold field electron sources produce beams of electrons with high brightness and low energy spread, and beams of electrons from cold field electron sources can be focused into small spots with high current densities. A major limitation of cold field electron sources is the poor stability of the electron beam current. Silicon cold field emitters without any coating or silicon cold field emitters with a metal coating do not produce sufficiently stable electron beams to be used in many applications. Schottky emitters are currently used as alternatives to cold field emitters in many tools (SEMs for example) because Schottky emitters emit current stably even though they have lower brightness and higher energy spread compared to cold field emitters.
Therefore, an improved electron emitter is needed